Conventionally, there is an image reading apparatus using a linear image sensor.
FIG. 24 shows a configuration of a linear CCD image sensor used in a conventional image reading apparatus.
Referring to FIG. 24, reference numeral 101 denotes a photoreceptive pixel array of the linear CCD image sensor; 102 and 103, analog shift registers for sequentially reading charges stored in the odd- and even-number pixels of the photoreceptive pixel array 101, respectively; and 104 and 105, output amplifiers for converting the charges read from the analog shift registers 102 and 103 into voltage signals and outputs the signals.
FIG. 25 shows an example of a configuration of an image reading apparatus which uses a linear image sensor of the aforesaid type (referred to as “E/O type linear sensor”, hereinafter).
Referring to FIG. 25, reference numeral 208 denotes a linear image sensor (here, an E/O type linear sensor); 201, a platen glass; 202, an original; 203, an illumination lamp for illuminating the original; 204 to 206, first to third mirrors, respectively; 207, a lens for forming an image of the original on the photoreceptive surface of the E/O type linear sensor 208; 209, an image sensor operation circuit for operating the E/O type linear sensor 208; and 210, a white board to be read for obtaining reference data used in shading correction processing.
The illumination lamp 203 and the first to third mirrors 204 to 206 are at the position indicated by the solid lines when performing a normal reading operation of reading the original, whereas, they moves to the position indicated by the dot lines when reading the white board 210. Further, the first to third mirrors 204 to 206 move in the sub-scanning direction S when reading the original, thereby reading the original in two dimensions.
It should be noted that, in the E/O type linear sensor as shown in FIG. 24, the reason of separately reading charges accumulated in the even- and odd-number pixels of the photoreceptive pixel array 101 is that there is a limitation in transfer speed in the analog shift registers 102 and 103, and it is necessary to do so to achieve a scan speed faster than a predetermined speed.
Recently, a demand for an image reading apparatus which achieves an even faster scan speed than ever is increasing, and a scan speed which can not be achieved by an E/O type linear sensor as shown in FIG. 24 is demanded.
Under the above described circumstance, as a linear CCD image sensor capable of achieving a scan speed twice faster than that of the conventional E/O type linear sensor, the one which separately outputs charges, from a photoreceptive pixel array, accumulated in the even-number pixels in the right-side area, charges accumulated in the odd-number pixels in the right-side area, charges accumulated in the even-number pixels in the left-side area, and charges accumulated in odd-number pixels in the left-side area (referred to as “R/L type linear sensor” hereinafter) has been suggested.
In FIG. 26, reference numeral 301 denotes a photoreceptive pixel array of the linear CCD image sensor; 302 to 305, analog shift registers for sequentially reading the charges accumulated in the even- and odd-number pixels in the right- and left-side areas of the photoreceptive pixel array 301; and 306 to 309, output amplifiers for converting the charges read out from the analog shift registers 302, 304, 303 and 305, respectively, into voltage signals and outputting them.
The analog shift registers 302 to 305 of the R/L type linear sensor shown in FIG. 26 respectively read out charges accumulated in the odd-number pixels in the left-side area, charges accumulated in the odd-number pixels in the right-side area, charges accumulated in the even-number pixels in the left-side area, and charges accumulated in the even-number pixels in the right-side area of the photoreceptive pixel array 301.
FIG. 27 shows a timing chart of operation signals for the R/L type linear sensor shown in FIG. 26 and the output signals from the R/L type linear sensor.
Referring to FIG. 27, “SH” shows a charge shift pulse and controls gates for simultaneously transferring charges accumulated in the photoreceptive pixel array 301 to the analog shift registers 302 to 305. Therefore, as shown in FIG. 27, a period between one SH pulse and the next SH pulse is an accumulation period (Tint) for accumulating charges in the photoreceptive pixel array 301.
Further, Φ1 and Φ2 in FIG. 27 are charge transfer pulses for operating the analog shift registers 302 to 305, and sequentially transferring the charges, by pixel, which has been simultaneously transferred from the photoreceptive pixel array 301 to the analog shift registers 302 to 305 by the SH pulse toward the output amplifiers 306 to 309 arranged at the end of each of the analog shift registers 302 to 305. As a result, image signals indicated by ODD-1, EVEN-1, ODD-2, and EVEN-2 are outputted.
However, when the R/L type linear sensor as shown in FIG. 26 is used, even slight differences in linearity among the four channels cause differences in level in the output signals separately read for the right- and left-side areas, which results in a discrepancy in read signals at the dividing point between the right- and left-side areas.
In the E/O type linear sensor, differences in signal levels between even- and odd-number pixels are observed merely as a very small repeating pattern in the image. In contrast, in the R/L type linear sensor, even a small difference in signal level is quite noticeable because of the discrepancy at the dividing point between the right- and left-side areas.